In an optical circuit, which makes use of an optical waveguide such as an optical fiber or planar optical waveguide, it is sometimes desirable to control the optical intensity of signal light to a suitable value, such as keeping the optical intensity constant for signal light transmitted through optical waveguides. In such a case, the optical intensity of the signal light is monitored in the optical circuit, and sometimes the optical intensity is also controlled on the basis of the results of this monitoring.